1. Field of the Invention
The invention relates to systems for using mechanical power to operate a hybrid electric vehicle.
2. Background Art
A hybrid electric vehicle (HEV) uses power generated by an internal combustion engine and an electric motor to move the vehicle. A HEV may provide improved fuel economy compared to conventional vehicles. One technique for improving the fuel economy in a HEV is to shutdown the engine when it is not needed to move the vehicle. In this situation, the electric motor provides power to move the vehicle. The engine then is started as needed. Starting the engine when the vehicle is moving under the power of the electric motor may cause a noticeable torque disturbance in a driveline of the vehicle.
Various methods are used to control an engine start in a vehicle. U.S. Pat. No. 7,013,213 to McGee et al. is an example of such a method. According to McGee et al., a smoothness factor is calculated to determine how smooth an engine start should be. Engine operating variables regulate engine smoothness during engine start events as determined by the calculated smoothness factor appropriate for selected vehicle operating conditions.
U.S. Pub. No. 2006/0137921 to Colvin et al. is another example of such a method. According to Colvin et al., a vehicle has a motor/generator, a disconnect clutch disposed between an engine and the motor/generator, and a transmission disposed between the motor/generator and vehicle drive wheels. The transmission includes an input clutch, which is selectively engagable for providing torque transfer between the motor/generator and the vehicle drive wheels. When an engine start is requested, the motor/generator is operated, and a start mode for the engine is determined based on a number of vehicle parameters. A transmission input clutch is partially disengaged to at least partially isolate the vehicle drive wheels from engine torque disturbances when the engine is started. The disconnect clutch then is engaged, and the engine is fueled to effect torque production by the engine.